A strong narrow peak in the field dependence of the radio-frequency absorption (the magnetic pseudoresonance) has been found and investigated in epitaxial thin films of La 2/3 Sr 1/3 MnO 3 possessing uniaxial magnetic anisotropy in the film plane. The peak is observed when the in-plane external magnetic field H is directed perpendicular to the easy axis and equals to the anisotropy field H u . The frequency dependence of the peak magnitude measured in the frequency range of 10-300 MHz approximately follows the Debye law behavior with the characteristic relaxation time of about 2.2 ns. The physical model of the phenomenon is suggested, based on the giant increase in static transversal susceptibility due to a sharp reorientation of the equilibrium magnetization when approaching the pseudoresonance conditions.
A spectrometer is described that ensures observation of NMR and relaxation in the effective field H2 acting in the doubly rotating frame (DRF). Unlike conventional NMR spectrometers, the presented apparatus allows the investigation of nuclear spin dynamics and relaxation under conditions of the specially transformed effective multispin dipole-dipole interaction Hamiltonian not studied up to now. The setup is a modified version of the previously described spectrometer for direct recording of NMR in the effective field H,1 in the singly rotating frame (RF), all its operating functions being retained. The new version of the apparatus is designed for the study of NMR and, especially, spin relaxation of 'H and 19F nuclei in solids under the magic-angle conditions in both the RF and the DRF. It enables one to obtain NMR spectra and longitudinal and transverse nuclear relaxation kinetics in the DRF field H,2 directly in their final form. In all operating modes, the apparatus is coherent with respect to the NMR frequency in the laboratory frame. In transient modes, it is also coherent with respect to the NMR frequencies in both the RF and the DRF. In all cases the NMR signals are recorded continuously during a single-scan experiment, as a rule not exceeding 1 s. The sensitivity and resolution capability of the spectrometer are superior to those of the starting one. In particular modes, its sensitivity is comparable with that of conventional NMR spectrometers. The apparatus enables one to investigate in detail slow molecular motions in solids with rates -7H,2 -103-105 s-' and = 7H, 1 -1 O-1 fl7 s -'; various motion parameters, including multiparticle correlations as well as a motion mechanism, can be extracted from the experimental data.
Proton magnetic spin-lattice relaxation in the effective field H, acting in the doubly rotating frame (DRF) was first applied to the study of slow internal protein dynamics in the submillisecond range of correlation times in the solid state. In this method the local dipolar magnetic field is reduced by the magic-angle rotating-frame method so that the resonance frequency of the relaxation experiment may be set below the value of the local field. As a result, unachievable by the standard nuclear magnetic resonance (NMR) relaxation techniques, slow molecular motions become experimentally accessible. The second effective field H2 is produced by the shallow sine-wave phase modulation of the H, pulse. The registration of the DRF spin-lattice relaxation signal takes place directly during the continuous H, pulse by means of an additional low-frequency radio-frequency coil oriented along the Ho field and operating at the rotating-frame NMR frequency of 100 kHz. The measurements of the spin-lattice relaxation time in the DRF within a wide temperature range have been performed in dry and hydrated ot-crystallin powders. This is the major protein in the eye lens, which prevents the uncontrolled aggregation of proteins and keeps the lens transparent. The results demonstrate that the protein hydration does not change the amplitude of slow side-chain motions but significantly shortens its correlation time: from about 50 to about 0.5 ss in dry and hydrated samples, respectively. The hydration also decreases the activation energy and restricts the distribution of the correlation times.
It is well known that moiecular motions in solids can b e monitored by their effect on the NMR linewidth or the transverse relaxation time T2 on the condition that 1) prospekt K a r h M:rrlis:~, 1 % 907 Moscow GSp-3, USSR.
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